It is known to use sodium gluconate as a regulating agent in concrete, and up until the present invention it has been used in relatively pure form. Sodium gluconate is a sodium salt of gluconic acid, which may be produced from glucose by fermentation. Glucose fermentation processes for the production of gluconates and gluconic acid are known. Microorganisms such as Aspergillus (hereinafter referred to as “A. niger”), Acetobacter, Bacillus, Pseudomonas, Gluconobacter, and Penicillium are known to be able to oxidize glucose into gluconates and gluconic acid. See e.g., World Patent Application WO/92/18637 of Asakura et al., page 2, lines 10-15.
It is hypothesized by Milson and Meers that the steps by which Aspergillus niger, A. suboxydans or Ps ovalis produce gluconic acid or gluconates are relatively few in number, consisting of the conversion of glucose to β-D-gluco-pyranose, and subsequent removal of two hydrogen atoms from β-D-gluco-pyranose to yield D-glucono-δ-lactone, and the hydrolysis of the latter to gluconic acid or a gluconate. (See P. E. Milson and J. L. Meers, “Gluconic and Itaconic Acids,” Comprehensive Biotechnology (Pergomon Press Ltd., Oxford, England 1985), Page 687, Chapter 35.4).
FIG. 1 illustrates a layout of a plant suitable for sodium gluconate and gluconic acid production using a microorganism such as A. niger. (See P. E. Milson and J. L. Meers, “Gluconic and Itaconic Acids,” Comprehensive Biotechnology (Pergomon Press Ltd., Oxford, England 1985), Page 684, Chapter 35.3.3). The prepared medium, which may include glucose, nutrients, and water, is sterilized continuously before the fermentation stage. Sterilization can alternatively be achieved in a fermenter, but this could lead to excessive darkening of the medium, and thus it is preferred to sterilize the medium outside the fermenter, which may be steamed and thus separately sterilized.
The nutrients used for gluconate production are typically carbohydrate sources including glucose either in the form of glucose monohydrate crystals or dextrose syrup. Additionally, sources of nitrogen (such as ammonium salts, urea, corn steep liquor), phosphate, potassium, and magnesium help to grow the microorganisms. It has been said that crude nitrogen and phosphate sources such as corn steep liquor can be avoided to minimize excessive cell growth which could in turn decrease gluconate production. In the absence of such sources, however, it is necessary to include trace amounts of iron, copper, and zinc in the medium.
The first stage of fermentation is the growth of vegetative inoculum. A suspension of cultured and selected Aspergillus niger (e.g., NRRL 3) is introduced into the inoculum fermenter, and initial pH is adjusted to about 6.5 with sodium hydroxide. Both the inoculum fermenter and production fermenter are typically stainless steel tanks that are stirred, baffled, and sparged with air. Antifoaming agents can be fed continuously into both fermenters to counteract foaming due to the sparging operation.
The second stage is “product fermentation,” namely, the fermentation of the glucose medium using the inoculum from the first stage. After spore germination and growth of mycelium at 30-33° C., inoculum is transferred to the production fermenter at a rate of about 1 part inoculum to about 10 parts production medium. Transfer may occur when a sufficient amount of mycelium is grown, or when an increase in glucose oxidase activity is detected in the mycelium. The medium in the production fermenter may contain glucose in a concentration of, for example, 220 kg glucose m−3, and its pH can be maintained automatically (˜6-7 pH) by monitoring with a pH electrode, which is preferably sterilizable, and adding sodium hydroxide (NaOH) as necessary.
The fermentation broth is usually kept at about 30-33° C. and agitated by sparging with air (e.g., 1.5 volume air/1.0 volume medium/minute), preferably with back pressure up to 2 bar on the production fermenter. The progress of gluconate production may be gauged by observing the rate of sodium hydroxide addition. Fermentation can be completed in as little as 19 hours.
The second (production) fermentation stage yields a crude intermediate fermentation broth that is then subjected during industrial process to a number of purification steps. The purification steps include the use of filtration, a decolorizer, and a clarifier to obtain gluconate or gluconic acid in substantially pure form. The purification steps are illustrated in FIG. 1 and discussed below.
In the purification step designated “broth filter” in FIG. 1, the fermentation broth is filtered (and/or otherwise centrifuged) to remove mycelium cell material, which is consequently destroyed by incineration or otherwise removed. The mycelium can be discarded or re-used. At this point, the broth contains minimal residual glucose.
In the next purification step designated “decolorizer” in FIG. 1, the fermentation broth is then passed through activated carbon to remove color, which is believed to be due to protein and other materials.
In the subsequent purification step designated “clarifier” in FIG. 1, the fermentation broth is clarified to remove fine particles from the fluid suspension, such as through the use of centrifugation.
After all three purification stages, the fermentation broth is evaporated to increase gluconic acid/gluconate concentration. If a 50% gluconic acid solution is desired, the broth is taken from the evaporator and passed through a cation exchanger to remove sodium ions (FIG. 1). If a technical grade of sodium gluconate (e.g., 98% purity) is desired, the liquor is dried directly by spray drying. If a pure grade of sodium gluconate is desired, the concentrated liquor is transferred from the evaporator to a crystallizer. An appropriate organic solvent, such as ethanol, may then be added, whereupon D-gluconic acid crystals can be obtained in salt form. The gluconate crystals are centrifuged to separate them from the so-called “mother liquor.”
This mother liquor, containing gluconate in the amount of about 60-75% based on total solids in the composition, may be re-circulated back to an earlier process step, or may be used as a liquid concrete admixture, as has been the customary practice in the concrete industry.